UPDATED 2:25 p.m. ET: Washington, DC Mayor Vincent Gray says at least a dozen people have been killed in the shooting. Authorities also say one shooter was killed, while they continue the manhunt for two other people who may have been involved. No motive is known at this time.

UPDATED 11:50 a.m. ET: CNN reports two suspects "are down." It's not clear whether that means they have been taken into custody or killed, although the Associated Press is reporting one shooter has been killed.

UPDATED 11:15 a.m. ET: Police report eight civilians are among the 10 who have reportedly been shot. CBS reports three people have died, with additional fatalities a possibility.There are also reports that multiple shooters were involved.